For this week we stay with the auxiliary data set and have a look at a periodic variable that has been mis-classified in the literature, has the wrong period, and we don’t even know which star is the variable one. But let’s start at the beginning:

A variable star has been identified by the ASAS-SN survey as ASASSN-V J210416.13+460844.0. They have measured a period of 0.1749958d (4hrs 11min 59.6sec) and classified the object as a high amplitude Delta-Scuti star. However, the variability in these stars is caused by the same pulsating mechanism that is at work in Delta-Cepheid stars. Thus, their light curves do look alike. Due to the much larger number of data points compared to ASAS-SN, it is obvious that the light curve has the wrong shape. Instead of the Delta-Cep shape, it looks very much like a sine-squared function. This is typically an indication of a contact binary, or WUMa-star. These are two stars in such a tight orbit that their surfaces do touch.

Accepting this, the period of the system needs to be double of the known one, as there have to be two minima per period. Hence, if one phase-folds the light curve with a period of 8hrs 23min 59s, one gets the plot as shown on the top of the page. A detailed investigation of the minima shows that they differ in brightness by 0.03mag. In other words, one of the stars is slightly brighter than the other.

Remains the question which star is the eclipsing binary. The ASAS-SN coordinates, and ours, point exactly between two almost equally bright stars. Higher resolution images show that they are separated by about 9 arcseconds on the sky. The ASAS-SN data and our auxiliary data set have a resolution of that order, hence the photometry of the two stars is merged. According to Gaia, one of the stars has a distance of 609pc, while the other one is at a distance of almost 2600pc. We will have to wait for the Gaia light curves to find out which of the stars is the eclipsing binary. Or, because the period is very short and variations can seen in  a few hours, observing the two stars for one night at a higher resolution should be able to make the decision. Note, that the merged photometry also means that the modelling of the system is tricky, as one needs to consider the contribution from the non variable star, which is changing the depth of the minima.

A very detailed look at the data reveals one further ‘feature’ of the object. The maximum of the light curve which follows the main minimum is 0.005mag brighter than the maximum that follows the secondary minimum. This O’Connell effect usually hints at asymmetric spot coverage on the surface of the stars. Despite the typical photometric scatter of 0.03mag in the individual data points, the huge amount of data allows us to measure these small differences in brightness reliably.